The present invention relates generally to gas turbine combustion liners and more specifically to a manner in which to improve the heat transfer through the combustion liner.
Gas turbine engines are a well-established form of technology capable of mechanical output in the form of propulsive thrust or shaft power for electrical generation. Gas turbine engines are well known in aircraft and power generation applications. A typical gas turbine engine comprises at least a compressor, a combustion system, and a turbine, which is coupled to the compressor. Depending on the power output required, the combustion system can comprise a single combustor or a plurality of combustors.
Within a gas turbine combustor, fuel is mixed with compressed air and introduced to an ignition source upon which combustion occurs, producing hot combustion gases that are directed to drive the turbine. This reaction of the fuel and compressed air mixture typically occurs in a combustion liner that is fabricated from high temperature capable material. However, material selection alone is not sufficient to maintain component life in the harsh combustion environment. Combustion liners also require thermal protection and dedicated cooling in order to maintain structural integrity during operation. With respect to thermal protection issues, this is typically accomplished by applying a thermal barrier coating to the inner surface of the combustion liner, that which is directly exposed to the combustion process. Typical thermal barrier coatings comprise a bond coat applied directly to the metal substrate and a top ceramic coat applied over the bond coat. An example of this type of coating is a well known and commercially available air plasma spray McrAlY bond coat followed by and air plasma sprayed Yttria Stabilized Zirconia (YSZ) Top Coat, where M is typically selected from a group comprising CoNi, NiCo, or Ni. Bond coat and top coat thickness will vary depending on the application. However, for a McrAly bond coat of 0.003″-0.005″ thick and Yttria Stabilized Zirconia top coat of 0.012″-0.018″ thick, the combustion liner manufacturer can estimate approximately a 100 degree F. benefit due to the coating, which is strongly dependent on thermal gradients. While coating a combustion liner provides some thermal benefits, it is not the only feature required to maintain structural integrity and component life of the combustion liner.
Another feature commonly used in combustion liners for accommodating the high operating temperatures is cooling schemes. More particularly, the placement of compressor discharge air not used in the combustion process, but instead used to cool the walls of the combustion liner. Compressed air can cool the combustion liner walls in a variety of manners including impingement cooling of the liner wall, being laid along the liner wall as a film of cooling air, and being directed through the combustion liner walls through elongated holes to cool throughout the material thickness as effusion cooling. The compressed air serves to provide a layer of air along the liner wall or throughout the liner wall that helps to reduce its effective operating temperature, so as to not exceed material capabilities.
Yet another manner in which combustion liners are protected from the harsh environment in which they operate is to augment the heat transfer through the combustion liner wall. This has been accomplished in the prior art by adding a coating to the liner wall backside such that cooling air passing over the outside of the liner wall is turbulated and heat transfer efficiency improved. An example of this type of heat transfer arrangement is disclosed in U.S. Pat. Nos. 6,526,756 and 6,546,730, hereby incorporated by reference. While this type of heat transfer augmentation is an improvement, it does not necessarily provide a uniform heat transfer improvement across all coated surfaces nor is a coating type heat transfer augmentation device preferable around cooling holes due to the processing requirements necessary to maintain open cooling holes and properly sized cooling holes.
What is needed is a means for augmenting the heat transfer across a combustion liner that provides uniform heat transfer across the entire selected area as well as a means that has little to no impact on other combustion liner cooling such as cooling hole definition and processing.